In large-scale steel structures such ships, marine structures, penstock, bridges, tanks, construction machines and the like, there has been a demand for enhancing strength of steel used therein so as to reduce the weight of the structure as a whole. In the steel material used in such steel structures, what is called “low alloy steel”, which contains less than 10 mass % or more preferably less than 5.0 mass % of alloy elements (Cr, Ni, Mo and the like),is mainly used. The strength of low alloy steel is generally in the range of 290 to 1180 MPa.
Due to the aforementioned demand for a steel material having higher strength, a low alloy steel material having high strength is preferably used. However, a low alloy steel material of such a type quite often experiences occurrence of cracking at a low temperature during welding. Such low-temperature cracking at welded joints of the high strength steel material is mainly caused by hydrogen which has been dissolved into the weld metal during the welding process. During the cooling the hydrogen diffuses and concentrates especially on the stress-concentrating portions such as the stop-end portion and the root portion which have been hardened by the welding heat, thereby generating cracks therein. It is well known that, the more such diffusive hydrogen is dissolved in the weld metal and the higher the stresses are, the more likely the low-temperature weld-cracking is to occur. It is also well known that, the larger the amount of the diffusive hydrogen is, the smaller stress is required to generate the low-temperature weld-cracking. The limit curve of the low-temperature weld-cracking generation is schematically shown in FIG. 2.
In order to prevent such low-temperature cracking during welding the following methods have been conventionally taken:    1) Pre-heating at the time of welding;    2) Appropriate post-heating immediately after welding;    3) Use of low hydrogen-type welding materials;
and with respect to the steel material as a material to be welded,    4) Use of a steel material in which carbon or the like is reduced to the low-carbon equivalent so as to reduce the weld-hardenability properties;    5) Use of a steel material in which PCM value is reduced so as to reduce the low-temperature weld-cracking sensitivities.
However, pre-heating and/or post-heating during welding is extremely painstaking and time-consuming work, which inevitably results in the higher cost for welding operation and lower efficiency of welding operation. In addition, in the case of high-strength steel of 780 MPa grade or higher, such high-tensile strength steel contains relatively large amounts of alloy elements which are added for ensuring higher strength, and thus essentially requires pre-heating during welding in terms of preventing the low-temperature cracking during welding, regardless of the use of low hydrogen type-welding materials.
In order to solve the aforementioned problems, JP-A 9-253860 Laid-Open, for example, proposes a welding method in which high-tensile strength steel of 760-980 N/mm2 grade is TIG welded by using a solid wire containing 7.5-12.0 mass % of Ni and 2 wt. ppm or less of H, at a wire supplying rate of 5 to 40 g/min, with the Ms point temperature temperature of the whole weld metals of 400° C. or lower. According to this welding method, although the high-tensile strength steel of 760 to 980 N/mm2 grade which has extra-thickness (more than 50 mm) is used, generation of weld-cracking at the room temperature can be prevented.
In addition, JP-A 11-138290 Laid-Open discloses a welding method in which a weld metal generated by welding experiences martensitic transformation during the cooling process after the welding so as to reach a state in which the weld metal is expanded, at the room temperature, as compared with the state thereof when the martensitic transformation started. The reference describes that an iron alloy whose martensitic transformation starting temperature is in the range of 170 to 250° C. (inclusive of 170° C. and exclusive of 250° C.) is used as the welding material.
However, the technique disclosed in JP-A 9-253860 Laid-Open is limited to the TIG welding. That is, there is a problem that, if other welding methods in which the amount of diffusive hydrogen is relatively large (2 wt. ppm or more) are employed in the technique, pre-heating will still be required in order to prevent weld-cracking.
In addition, in the technique described in JP-A 11-138290 Laid-Open, there arises what is called the “over-matching” problem in which the strength of the weld metal becomes larger than the strength of the welded materials.